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When exercise is undertaken in warm, humid conditions, the thermal gradient between the skin and environment, and the capacity for evaporative heat loss, are reduced. These factors, along with an increase in metabolic heat production, lower work capacity and exercise performance. Thermoreceptors located within the skin and deep in the body convey information on this accumulation of thermal energy to higher brain structures and, if mean body temperature rises uncontrollably, the cumulative neuronal input is thought to produce inhibitory signals that lower work capacity, such that metabolic heat production decreases to protect the organism from heat injury. Lessening these inhibitory signals may enhance or help to maintain exercise performance in the heat. The inhibitory signals might be lessened by cooling the skin and deep body temperature prior to or during exercise, or perhaps by applying menthol on the skin, or some combination of these.

Menthol is a chemical compound that activates cold receptors (TRPM8) in the skin to elicit cool sensations. These receptors are not otherwise activated unless cooled below 27 C. Hence, menthol, when applied to the skin of heat stressed humans, may provide a “cool’’neuronal input to higher brain structures in addition to the neuronal signals arising from warm thermoreceptors located within the body. But menthol may also induce a heat storage (cold defense) response that would then heighten the activity of warm receptors deep in the body. Therefore, it is not clear whether menthol might reduce, enhance or help to maintain exercise performance in heat stressed humans. Moreover, no studies have …